1. Field of the Invention
The present invention relates to a pressure sensor for monitoring, such as, an air pressure in a tire of an automotive vehicle.
2. Description of the Prior Art
According to one type of a conventional pressure sensor as disclosed, such as, in Japanese First (unexamined) Patent Publication No. 54-124771, a piezoelectric element is used for monitoring a pressure variation. Specifically, the piezoelectric element is arranged to vary its electrical capacitance depending on a pressure applied thereto from a diaphragm or bellows which is arranged to displace according to a pressure differential applied thereacross. This variation of the electrical capacitance of the piezoelectric element is converted to an electric signal so as to electrically detect the monitored pressure variation.
Another type of the pressure sensor has been proposed which utilizes a piezoresistant element mounted on a diaphragm so as to detect a pressure variation as a variation of an electric resistance of the piezoresistant element.
In the former conventional pressure sensor, however, the material characteristic of the piezoelectric element is an essential factor for determining the pressure-capacitance characteristic thereof, i.e. the detection sensitivity of the pressure sensor. As a result, there has been a problem in that uneven individual characteristics or qualities of manufactured piezoelectric elements cause uneven detection sensitivities of the pressure sensors, leading to measurement errors in pressure.
Similarly, this problem also applies to the latter conventional pressure sensor, wherein the material characteristic of the piezoresistant element is an essential factor for determining the measurement precision or accuracy of the pressure sensor.